1. Field of the Invention
The present invention relates to an alloy type thermal fuse and a wire member for a thermal fuse element, and is useful as a thermoprotector for an electrical appliance or a circuit element.
An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor.
Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator.
The alloy type thermal fuse has the following operation mechanism.
The alloy type thermal fuse is disposed so as to thermally contact an electrical appliance or a circuit element which is to be protected. When the electrical appliance or the circuit element is caused to generate heat by any ab-normality, the fuse element alloy of the thermal fuse is melted by the generated heat, and the molten alloy is divided and spheroidized because of the wettability with respect to a lead conductor or an electrode under the coexistence with the flux that has already melted. The power supply is finally interrupted as a result of advancement of the division and spheroidization. The temperature of the appliance is lowered by the power supply interruption, and the divided molten alloys are solidified, whereby the non-return cut-off operation is completed. In an alloy type thermal fuse, therefore, it is requested that the division temperature of the fuse element alloy is substantially equal to the allowable temperature of an electrical appliance or the like.
Usually, a low-melting alloy is used as such a fuse element. As apparent from a phase equilibrium diagram, an alloy has a solidus temperature and a liquidus temperature, and, at the eutectic point where the solidus temperature coincides with the liquidus temperature, the alloy is changed all at once from the solid phase to the liquid phase by heating which causes the alloy to pass the eutectic temperature. By contrast, in a composition other than the eutectic point, an alloy is changed in the sequence of the solid phase→the solid-liquid coexisting phase→the liquid phase, and the solid-liquid coexisting region temperature width ΔT exists between the solidus temperature Ts and the liquidus temperature Tl. Even in the solid-liquid coexisting region, there is the possibility that the division of a fuse element occurs, although the possibility is low. In order to reduce the dispersion of the operating temperature among thermal fuses, it is requested to use an alloy composition in which the solid-liquid coexisting region temperature width ΔT is as narrow as possible. One of conditions imposed on an alloy type thermal fuse is that ΔT is narrow.
When ΔT is large, the following disadvantage is caused in addition to the above-mentioned large dispersion of the operating temperature. In the case where the upper limit temperature of a normal heat cycle reaches the solidus temperature, even when a fuse element is not broken in the heat cycle, the initial state of a semi-molten state (solid-liquid coexisting state) occurs. During a temperature lowering process in a heat cycle, the alloy is resolidified. The repetition of the semi-melting and the resolidification causes the operation characteristic to be disturbed, so that the operation stability to a heat cycle is impaired.
Even when the solidus temperature is not lower than the upper limit temperature of a normal heat cycle, a large slip which may be caused in the interface between different phases in the alloy structure is increased depending on the ductility of the fuse element. Such a slip is repeatedly caused as a result of a heat cycle, so that a change of a sectional area or an elongation of the element occurs in an excessive manner. From this point of view, the operation stability to a heat cycle cannot be often assured.
In many cases, a fuse element of an alloy type thermal fuse is used in the form of a linear piece. In order to reduce the size of a thermal fuse so as to comply with the recent tendency that appliances are further miniaturized, it is sometimes demanded to realize a thin fuse element. A fuse element is often requested to have drawability to a small diameter (for example, 400 μmφ or smaller).
Another one of the conditions imposed on an alloy type thermal fuse is that the electrical resistance is low. The temperature rise of a fuse element by Joule's heat in a normal state is indicated by ΔT′. The operating temperature is lower than that in the case where such a temperature rise does not occur. As ΔT′ is higher, the operation error is larger. In order to suppress Joule's heat, therefore, a fuse element is requested to have a low specific resistance. The resistance of a fuse element is inversely proportional to the sectional area of the fuse element. In order to meet the requirement of thinning, therefore, a fuse element is requested to have a lower specific resistance.
In recent electrical appliances, the use of materials harmful to a living body, particularly metals such as Pb, Cd, Hg, and Tl is restricted because of increased awareness of environment conservation. Also a fuse element for a thermal fuse is requested not to contain such a harmful metal.
2. Description of the Prior Art
When alloy type thermal fuses are classified according to operating temperature, thermal fuses of an operating temperature of about 150° C. are widely used.
Such a thermal fuse, known are a thermal fuse in which an alloy of 49.8Sn-31.96Pb-18.11Cd (the weight composition of the alloy of 49.8% Sn, 31.96% Pb, and 18.11% Cd, this indication method of an alloy composition is employed in the following description) is used as a fuse element, and which has an operating temperature of 145° C. (Japanese Patent Application Laying-Open No. 57-58011), and that in which an alloy of 54Sn-25Pb-21In is used as a fuse element, and which has an operating temperature of 145° C. (Japanese Patent Application Laying-Open No. 59-8231). However, these thermal fuses contain harmful metals such as Cd and Pb, and cannot satisfy the above-mentioned requirements for environment conservation. Also a thermal fuse of an operating temperature of 135 to 145° C. in which 0.1 to 5 weight parts of Ag are mixed to 100 weight parts of an alloy of 1 to 3 Sn-balance In is known (Japanese Patent Application Laying-Open No. 2002-25404). The fuse element contains a large amount of In which is a highly reactive element. Therefore, In in the alloy surface reacts with a flux to be dissolved into the flux surrounding the fuse element. When this is repeated, the alloy composition of the fuse element is changed in the direction of a reduction of the amount of In, and the function of the flux is lowered, so that the operation performance of the fuse element is inevitably changed with age. After an elapse of a long term, therefore, the fuse element cannot be assured to perform a predetermined operation performance.
In an alloy for a fuse element of an operating temperature of about 150° C., it is requested that its liquidus temperature is approximately 150° C. Various alloys which satisfy the requirement that a fuse element is free from a harmful metal, in addition to the temperature requirement are known. In these alloys, however, the above-mentioned solid-liquid coexisting region temperature width ΔT is large, and the above-mentioned requirements such as the reduced dispersion of the operating temperature, and the operation stability to a heat cycle are hardly satisfied. In 50Bi-50Sn, for example, the liquidus temperature is about 154° C., and a harmful metal is not contained. In a Bi—Sn alloy, the solidus temperature is constant or 139° C., and the solid-liquid coexisting region temperature width ΔT is as large as about 15° C., so that the requirements cannot be sufficiently satisfied.